Dogfooding: use container scanning on GitLab omnibus

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As mentioned in #9909 (closed), it would be great to utilize more Secure features when building GitLab's omnibus package. From the related issue:

• omnibus should run container_scanning. Even if we check the dependencies with https://gitlab.com/gitlab-org/security-products/gitlab-depscan, it's only the first level deps. Clair will scan all the layers, so if a transient dep is installed, it could be spotted there.